Since polyaniline has better electrically conducting property when doped, additionally, is easily obtained from an inexpensive aniline polymer simply and at a high yield, and is chemically stable, it is being studied in a variety of fields, and application development thereof is being tried. In order that polyaniline exhibits electrical conductivity, as shown in FIG. 2, generally, it is necessary that polyaniline in a non-electrically conductive base form undergo protonation with a dopant. However, since in polyaniline in a non-electrically conductive base form, a p-conjugation system is generally developed, polyaniline is insoluble in almost all organic solvents and water, and, therefore, protonation itself is difficult.
As a dopant for polyaniline, generally, in order to effectively perform protonation, a sulfate ion (e.g. JP-A H02-233701), as well as a strong acid anion such as a hydrochloride ion, a nitrate ion and a phosphate ion are used. In addition, an organic dopant is also known (Patent Document 2). It is used as a counterion. To the contrary, as a dopant derived from lignin which is a forest carbon source, utilization of general sulfonated lignin as industrial lignin is disclosed (e.g. European Polymer Journal 37 (2001) 2217-2223, European Polymer Journal 38 (2002) 2213-2217.